Embodiments relate to transmitting channel state information (CSI) and/or acknowledgement information in a wireless network, for example, a Long Term Evolution (LTE) wireless network.
The 3GPP Long Term Evolution (LTE) represents a major advance in cellular technology. LTE is designed to meet carrier needs for high-speed data and media transport as well as high-capacity voice support well into the next decade. LTE encompasses high-speed data, multimedia unicast and multimedia broadcast services.
The LTE physical layer (PHY) is an efficient mechanism for conveying both data and control information between an enhanced base station (e-NodeB) and mobile user equipment (UE). The LTE PHY employs some advanced technologies to cellular applications. These technologies include Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO) data transmission. In addition, the LTE PHY uses Orthogonal Frequency Division Multiple Access (OFDMA) on the downlink (DL) and Single Carrier-Frequency Division Multiple Access (SC-FDMA) on the uplink (UL). OFDMA allows data to be directed to or from multiple users on a subcarrier-by-subcarrier basis for a specified number of symbol periods.
In 3GPP LTE Rel-10 carrier aggregation (CA), dropping of CSI that is configured to be sent periodically occurs in the following cases: (1) when multiple Channel State Information (CSI) report(s) collide in the same sub-frame, only one CSI report is transmitted, and the others are dropped; and (2) when CSI report(s) collides with Hybrid Automatic Repeat Request-Acknowledgement (HARQ-ACK) in the same sub-frame, CSI report(s) are dropped.
Enhancements are desirable to improve or avoid CSI dropping in CA. One option that has been proposed is to use a data channel, the Physical Uplink Shared Channel (PUSCH), to carry CSI that is configured to be sent periodically. Since PUSCH can carry a much bigger payload than Physical Uplink Control Channel PUCCH, multiple CSI reports (up to 5 component carriers, as defined in CA currently) can be carried in the same sub-frame without any report being dropped.
The problem is how to make best use of PUSCH to provide as much useful CSI information as possible in an efficient way, given that the smallest unit of resource that can be allocated on PUSCH is significantly larger than the amount of CSI that is typically transmitted periodically.
There are a few known solutions on how to use PUSCH to carry CSI:
(1) The e-NodeB can request the UE to report a single individual aperiodic CSI report (which conveys more CSI than a periodic one). This is already supported in Rel-10. The drawback is that for each aperiodic CSI request, the e-NodeB needs to send a Downlink Control Information (DCI) message on Physical Downlink Control Channel (PDCCH). This can result in significant overhead on PDCCH.
(2) Use higher layer signaling to configure periodic resource on PUSCH, and reuse existing periodic reporting modes. Existing periodic modes have relatively smaller payload size, up to 55 bits for 5 component carriers. This can be an inefficient way of using one Physical Resource Block (PRB) on PUSCH since one PRB is significantly larger than this.
(3) Define a different reporting mode for periodic CSI on PUSCH with more comprehensive CSI feedback (thus larger payload) in each sub-frame. One possible way is to reuse the existing aperiodic reporting formats, but sent periodically instead of as single individually-requested reports. This can better take advantage of the one-PRB resource on PUSCH. The disadvantage is that when the CSI reports occur together with UL data, CSI needs to be multiplexed with UL data, and when the CSI payload is large, the impact on UL data is larger because it takes more resource away from UL data.
(4) Largely reuse the aperiodic CSI mechanism in 3GPP LTE Rel-10 (as in the first option), but allow dynamic triggering of small or large payload size CSI reporting. This is achieved by re-defining the meaning of CSI request field in the DCI format. It has the same drawback as the first option, namely, large overhead on PDCCH.
In 3GPP LTE Rel-10, HARQ-ACK bundling, either in time domain or spatial domain, is applied in certain cases in order to reduce the payload size of HARQ-ACK to fit into the PUCCH transmission formats. When HARQ-ACK bundling is performed, there is a loss in downlink throughput as a result of some unnecessary negative acknowledgements due to bundling.
Both CSI and HARQ-ACK are considered as uplink control information (UCI).